Is this issue on-going or does it happen from time to time?
I have seen an issue where 3 or so sites can be accessed but that is it. First and foremost you want to run these commands on your pc:
ipconfig /release
ipconfig/renew
ipconfig/flushdns
That will fix many network related issue's.
If those don't seem to clear up the issues please post back here as we can dig further...Oh, also try a PC restart ;)
Clear cache, cookies, temp files, etc in you browser settings.

Edit: Also verify you are on the correct wireless network..While which network you are on shouldn't make a difference, it might clear networking issues up.